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• Introductory Note.
• Book I.
• Chap. I.: Whether a Demonstrative Science Exists
• Chap. II.: What Knowing Is, What Demonstration Is, and of What It Consists
• Chap. III.: A Refutation of the Error Into Which Some Have Fallen Concerning Science and Demonstration
• Chap. IV.: The Meaning of ‘distributive,’ ‘essential,’ ‘universal’
• Chap. V.: From What Causes Mistakes Arise With Regard to the Discovery of the Universal. How They May Be Avoided
• Chap. VI.: Demonstration Is Founded On Necessary and Essential Principles
• Chap. VII.: The Premises and the Conclusion of a Demonstration Must Belong to the Same Genus
• Chap. VIII.: Demonstration Is Concerned Only With What Is Eternal
• Chap. IX.: Demonstration Is Founded Not On General, But On Special and Indemonstrable Principles; Nor Is It Easy to Know Whether One Really Possesses Knowledge Drawn From These Principles
• Chap. X.: The Definition and Division of Principles
• Chap. XI.: On Certain Principles Which Are Common to All Sciences
• Chap. XII.: On Questions, And, In Passing, On the Way In Which Sciences Are Extended
• Chap. XIII.: The Difference Between the Demonstration and Science of a Thing’s Nature and Those of Its Cause
• Chap. XIV.: The Figure Proper to Demonstrate Syllogism
• Chap. XV.: On Immediate Negative Propositions
• Chap. XVI.: On Ignorance Resulting From a Defective Arrangement of Terms In Mediate Propositions
• Chap. XVII.: On Ignorance Resulting From a Defective Arrangement of Terms In Immediate Propositions
• Chap. XVIII.: On Ignorance As Resulting From Defective Sense Perception
• Chap. XIX.: Whether the Principles of Demonstration Are Finite Or Infinite
• Chap. XX.: Middle Terms Are Not Infinite
• Chap. XXI.: In Negations Some Final and Ultimate Point Is Reached Where the Series Must Cease
• Chap. XXII.: In Affirmations Some Final and Ultimate Point Is Reached Where the Series Must Cease
• Chap. XXIII.: Certain Corollaries
• Chap. XXIV.: Whether Universal Or Particular Demonstration Is Superior
• Chap. XXV.: That Affirmative Is Superior to Negative Demonstration
• Chap. XXVI.: Direct Demonstration Is Superior to Reduction Per Impossible
• Chap. XXVII.: What Science Is More Certain and Prior, and What Less Certain and Inferior
• Chap. XXVIII.: What Constitutes One Or Many Sciences
• Chap. XXIX.: Concerning Many Demonstrations of the Same Thing
• Chap. XXX.: On Fortuitous Occurrences
• Chap. XXXI.: Sense Perception Cannot Give Demonstrative Science
• Chap. XXXII.: On the Difference of Principles Corresponding to the Difference of Syllogisms
• Chap. XXXIII.: The Distinction Between Science and Opinion
• Chap. XXXIV.: On Sagacity
• Book II.
• Chap. I.: On the Number and Arrangements of Questions
• Chap. II.: Every Question Is Concerned With the Discovery of a Middle Term
• Chap. III.: The Distinction Between Definition and Demonstration
• Chap. IV.: The Essence of a Thing Cannot Be Attained By Syllogism
• Chap. V.: Knowledge of the Essence Cannot Be Attained By Division
• Chap. VI.: The Essence Cannot Be Proved By the Definition of the Thing Itself Or By That of Its Opposite
• Chap. VII.: Whether the Essence Can In Any Way Be Proved
• Chap. VIII.: How the Essence Can Be Proved
• Chap. IX.: What Essences Can and What Cannot Be Proved
• Chap. X.: The Nature and Forms of Definition
• Chap. XI.: The Kinds of Causes Used In Demonstration
• Chap. XII.: On the Causes of Events Which Exist, Are In Process, Have Happened, Or Will Happen
• Chap. XIII.: On the Search For a Definition
• Chap. XIV.: On the Discovery of Questions For Demonstration
• Chap. XV.: How Far the Same Middle Term Is Employed For Demonstrating Different Questions
• Chap. XVI.: On Inferring the Cause From the Effect
• Chap. XVII.: Whether There Can Be Several Causes of the Same Thing
• Chap. XVIII.: Which Is the Prior Cause, That Which Is Nearer the Particular, Or the More Universal?
• Chap. XIX.: On the Attainment of Primary Principles
• Appendix. Prior Analytics. Book II.
• Chap. XXIII.: On Induction
• XXIV.: On Example

CHAP. X.

The nature and forms of Definition

Since definition is, as we have said, the expression of a thing’s essence, it is clear that one kind of definition may give the meaning of the name, or of some other expression having the force of a name:—e.g. the definition of the meaning of ‘nature of triangularity as such.’ On learning that the thing corresponding to the name exists we enquire why it exists, for it is difficult to grasp the cause of a thing when we do not know previously that the thing exists. The reason of this difficulty has been mentioned before (II, c. 8), namely that we do not know whether a thing exists or not except accidentally.

An expression may have unity in two ways; either from a union of the separate parts (in the manner in which the whole Iliad is a unity), or from predicating an attribute of the subject essentially, not accidentally. Thus one form of definition is that which gives the meaning of a word, as mentioned above. Another explains the reason why a thing is. Hence the former explains signification but proves nothing, while the latter clearly gives a kind of demonstration of the subject’s essence, and differs from demonstration only in form. Thus there is a difference between saying, ‘Why does it thunder?’ and ‘What is thunder?’ To the former one would answer, ‘because fire is quenched in the clouds’; to the question, ‘What is thunder?’ ‘Thunder is the noise of fire being quenched in the clouds.’ Thus the same thought is expressed in two different ways; the former answer containing all the parts of a demonstration, the latter being stated as a definition.

Further there may be a definition of thunder, as ‘a noise in the clouds,’ which forms only the conclusion of the demonstration of the essence of thunder. Lastly the definition of ultimate terms is an indemonstrable proposition stating the essence of the subject. Consequently definition is either (1) An indemonstrable expression of the thing’s essence, or (2) A syllogism expressing its essence, differing only in form from demonstration, or (3) The conclusion of a demonstration which states the subject’s essence. What has been said is enough to shew how far a thing’s essence is demonstrable, how far the reverse; also what things admit of demonstration and what do not, in what senses ‘definition’ is used, and in what ways it does or does not prove a thing’s essence, and in what cases this can be done; lastly the relations of definition to demonstration have been given, and it has been shewn how far the two may have the same object.